A main controller (monitoring box) of a frequency converter has a unit-combined structure, and its core is a central processing unit (CPU) unit having double CPUs communicating with an interface board and phase control boards A, B and C via a bus. An operation command, a frequency setting signal, an electric machine current and voltage, and so on may be accepted from an analog input interface. A CPU board calculates control information and status information according to the operation command, the setting signal and other input signals. The phase control boards A, B and C accept the control information from the CPU board, generate pulse width modulation (PWM) control signals and send control optical signals to a power unit via an electric-to-optical transducer. An acknowledgement signal from the power unit is converted to an electric signal in the phase control boards A, B and C, subjected to processing and then sent to the CPU board for processing. The status information may be sent out through the interface board and an interface sub-template. Debugging of the main controller is very important for the high-voltage frequency converter, however, existing debugging means is simplified, and an effect of testing can not be achieved actually under self-operating by simulating the high voltage in a case of shielding relevant status information of the power unit. It is much more troublesome with being tested under high voltage condition due to hugeness of the equipment, inconvenience of connections of high voltage wires and loads and high cost, and more problems occur when there are faults and repair is needed.